Electrolytic electrophotography



y 2, 1967 A. F. KA SPAUL ET AL 3,317,409

ELECTROLYTI G EDECTROPHOTOGRAPHY Filed April 16, 1963 FIG.2

ALFR ED FKASPAUL-INVENTORS JOHN W. CHRISTENSEN- FM ASW TH ElR ATTORNEYUnited States Patent O 3,317,409 ELECTROLYTIC ELECTROPHOTOGRAPHY AlfredF. Kaspaul, Fairview, Pa., and John W. Christensen, New Canaan, Conn.,assignors to Minnesota Mining and Manufacturing Company Filed Apr. 16,1963, Ser. No. 273,471 6 Claims. (Cl. 204-18) This invention relates tothe production of images on photoconductive surfaces.

More particularly, this invention relates to a method for producingimages on photoconductive surfaces which have been precoated with alight transmissive layer of metal, to the images so produced, and alsoto methods for producing such metal precoated photoconductive surfaces.

The art has heretofore generally appreciated that photoconductivesurfaces can be used to record visible images by'selectively depositingimage-forming materials on a photoconductive surface bearing a latentimage by utilizing the difference in electrical conductivity betweenilluminated and non-illuminated areas. The general procedure is toemploy electrophotographic techniques and deposit material selectivelyupon photoconductive surface, either concurrently with or shortly afterthe exposure of such surface to a light image.

It is an object of this invention to provide a process for producingreproductions using photoconductive layers. Another object of theinvention is to provide a process for making positive prints. Anotherobject is to provide a photoconductive copy sheet which has a metallicimageforming-layer. Other objects will be apparent to those skilled inthe art from the accompanying disclosure.

In accordance with the objects of the invention, it has now beendiscovered that images can be formed on photoconductive surfaces byselectively removing metal from a photoconductive surface precoated witha light-transmissive film of metal, with the result that a visible imageis produced. Thus, for example, one can directly produce positive imageson a zinc oxide photoconductive surface by selectively removing metalfrom a thin precoated layer of metal upon such photoconductive surface.

In practicing the invention, strongly photoconductive material which hasbeen deposited in the form of a layer upon a conductive substrate iscoated with a uniform lighttransmissive film of metal over the entiresurface, to form a multi-layered light-sensitive sheet. The sheet isthen exposed to a light image of the matter to be recorded, for a timesuflicient to effect a change in the conductivity of the photoconductor.Thereafter, the latent image existing as areas of differentialconductivity in the photoconductor layer is developed byelectrolytically removing metal from the upper metallic layer.

Suitable semi-conductive photoconductive materials include highlyconductive cadmium sulfide, cadmium selenide, antimony sulfide (Sb Szinc oxide, zinc sulfide, indium oxide, and the like. Thephotoconductors should have electrical conductivity on exposure to lightof at least about mho per cm. and preferably between about 10* and 10*mho per cm. A preferred photoconductive material for purposes of thisinvention because of its inherent physical properties, such as its decaytime or light memory, is highly conductive zinc oxide.

Suitable photoconductive layers are conveniently produced by bonding tothe surface or the like of a conductive sheet, such as a metal sheet,which may be backed with paper, if desired. The photoconductor is mixedwith a suitable organic bonding agent and applied in admixture to thesurface of the sheet, and dried. Suitable bonding agents include acopolymer of styrene and butadiene known as Pliolite, polystyrene,chlorinated rubber, silicone resin, rubber hydrochloride, polyvinylidenechloride and the like. The binder should be a good insulator and asnonconductive as the photoconductor in the dark, having conductivity nogreater than 10- mho per cm. Preferably, the binders should be solublein conventional organic solvents so that they can be applied to thesurface in solution. The bonding agents should also have adequatebonding strength between the photoconductor and the support, andpreferably should be hydrophobic or Water-insoluble. Preferably, thephotoconductor layer is sufficiently conductive that a low voltage canbe used during development. Usually the thickness of the layer isbetween about 0.1 and about 5 mils, and preferably between about 0.5 andabout 2 mils.

Using vacuum vapor deposition techniques and preferably pressures notgreater than about 10- mm. Hg, the selected metal is vaporized andallowed to deposit upon the surface of the photoconductor. Enough metalis deposited to produce a thin layer which is about 10 to 90 percent andpreferably from about 40 to percent optically transmissive. Thinnermetal films can be used when the metal is dark colored. In general,transmissive metal films suitable for the purposes of the invention havethicknesses of from about to Angstrom units, depending on the metalused.

For purposes of this invention, light transmission through a metal layeron a photoconductive surface can be measured using a tungsten filamentlight source, by comparing the perpendicular transmission through anuncoated inch thick transparent glass plate to the same plate coated byvacuum vapor techniques with the particular metal being coated, themetal deposition upon such glass plate being carried out under the sameconditions as, and preferably simultaneously with, the deposition ofsuch metal upon the particular photoconductive surface being coated withthis metal.

One suitable method for coating a photoconductive surface by vacuumvapor deposition is as follows: Sheets of photoconductive paper, whichare to be coated with a layer of, say cobalt, are circumferentiallymounted on a rotatable stage in the bell jar of a vacuum apparatus.Between successive pieces of paper are placed the glass comparativeoptical transmission plates described above. Beneath an area near theedge of the stage where plates and sheets are circumferentially mountedis placed a heated source of cobalt vapor. Between the stage and thecobalt vapor source, a shutter arrangement is mounted which is suitablefor controlling the exposure time of the materials to be coated bycobalt vapor. Also beneath the stage at a distance from the cobalt vaporis mounted a tungsten filament light source so arranged that when thestage is rotating, paper and glass being vapor-coated pass verticallyover the filament. A photocell is positioned vertically above the stageover the tungsten filament. Cobalt deposits both upon the glass platesand upon the photoconductive sheets during coating. When the opticaltransmission of the tungsten light source through the glass is reducedto the desired level (say, 50 percent), compared to the orginaltransmission through the glass as measured by the photocell, the vapordeposition is discontinued.

In general, any conductive electrolyzable metal can be used for purposesof this invention, such as copper, vanadium, zinc, bismuth, cadmium,titanium, silver, nickel, and the like. Preferably, the metal is dark incolor for contrast; however, if readout of the image is to be made byother than optical viewing, such as by electronic readout or the like,even very thin, non-optically contrasting metal films are useful.Preferred metals for the purpose of the invention are cobalt, titaniumand vanadium.

In the case of metals which have heats of vaporization lower than about50 kilocalories per mole, an intermediate step is desirable andsometimes necessary before such metals are vapor-coated upon aphotoconductive substrate. This intermediate step comprises depositingupon the sur face to be metal-coated by vacuum vapor deposition anintermediate nucleating layer having from about 10 to 10 atoms persquare centimeter of a metal or metal alloy having a heat ofvaporization exceeding 0 kilocalories per mole. Such a layer isdiscontinuous and substantially optically transparent. The techniqueused is similar to that disclosed in United States Patent No. 2,754,230.

Such a preliminary deposition appears to create nucleation sites,thereby enhancing subsequent deposition and adherence of metals havingheat of vaporization lower than 50 kilocalories per mole so as toproduce metal films which are from about 40 to 70 percent opticallytransmissive.

It is theorized that, and there is evidence to substantiate that, metalfilms vapor-coated in accordance with the teachings of this inventionare not continuous but are rather in the form of little islets havingprobable separation from one another of, say, about Angstrom units. Thepresence of these metal islets in such amounts that the opticaltransmissivity of the film is in the range of about 40 to 70 percentproduces coatings which have little or no lateral electricalconductivity. Below about 40 percent they may form an electricallycontinuous surface giving rise to approximately bulk values ofconductivity.

In general, evaporated films display very low conductivity until acertain critical optical transmission level is reached, after which theconductivity increases very rapidly as a function of decrease of percentoptical transmission until values approaching conventional bulkconductivity values of the particular metal being vapor-coated areobtained. Naturally, the bulk conductivity value for any given metalfilm has to be determined empirically in each case since it varies withthe type of evaporated metal involved, residual gas pressure in thevacuum apparatus during vapor deposition, rate of metal vaporization,the surface condition of the particular substrate involved (e.g., thepresence of nucleation sites), and other factors. For example, vanadiumfilms on glass plates having an optical transmissivity of the order of20 percent or less have resistance of 100 ohms/square or less, whilefilms having 50 percent transmission have over 1000 ohms/ squareresistance. At 70 percent optical transmissivity, the resistance isabout 5000 ohms/ square.

It appears that for purposes of the present invention greatest opticalcontrast in a developed image is achieved by using evaporated metalfilms in which the maximum amount of metal vapor is deposited which isjust insufiicient to produce a bulk value for conductivity. Since bulkconductivity values are dependent upon so many variables, as mentionedabove, it is usually simpler and more expedient simply to depositsufiicient metal by vacuum vapor deposition upon a photoconductivesurface until there is produced a metal film having optical transmissioncharacteristics of from about 40 to 70 percent.

After the light-transmissive layer has been deposited on thephotoconductive layer, the multi-layer sheet is ready for use to producerecords of light images. The sheets are conveniently stored in theabsence of bright light so as to achieve the dark conductivity state. i

To make a reproduction of an image using the photoconductive sheetconstruction of the invention, the sheet is exposed to a radiationpattern or light image, for example, by means of daylight or anincandescent lamp. The radiation may be actinic light, ultravioletlight, or other radiation such as X-rays which effects a change in theconductivity of the photoconductor layer, herein referred to generallyand inclusively as light. The resulting differentially conductive imageis then developed to visible (readable) form.

To form a visible image the surface of the light-transmissive metal filmis contacted with a liquid or gel electrolyte, usually aqueous incharacter, and electrolyzed. The electrolyte itself need have no specialcharacteristics, but can be any soluble salt or salt-like material whichionizes in solution. Suitable electrolytes include CaSO Na SO NaCl, CoClKClO LiCl, NH Cl, Mg(NO NaNO MgCl Mg(ClO) KCl, or HCl. Preferredelectrolytes are those in which electrochemical oxidation of the metalresults in formation of a soluble salt of the metal, or an insolublesalt which is colorless or nearly transparent.

If the photoconductive surface upon'which the evaporated metal layerrests is one having a long decay time, that is, one which is capable ofstoring a latent image for a sufficiently long period of time afterremoval of the light source bearing the image to be recorded, then theelectrolyte solution can be contacted with the surface of the evaporatedmetal film after latent image formation before the image in thephotoconductive material has faded. However, if the photoconductivematerial has a very short decay time, then the electrolyte solutionneeeds to be in contact with the evaporated film over thephotoconductive surface during the time when the image-bearing light isprojected upon the photoconductive surface bearing the evaporated metallayer. In either situation, a current is made to flow through theelectrolyte solution. Usually an inert electrode in contact with theelectrolyte solution is made the cathode and the electrically conductivesubstrate beneath the photoconductive material is made the anode.

Those areas of the photoconductive material which are exposed to lightare more conductive than those which are not, with the general resultthat electron flow through regions which have been exposed to light isgreater than elsewhere on the photoconductive surface. The difference inconductivity is at least ten-fold, and generally as much as -fold ormore. The passage of electrons through the electrolyte solution, throughevaporated metal film, and through the photoconductive surface resultsin the selective removal of the evaporated metal layer upon thephotoconductive surface, with the effect that metal is oxidized awayfrom the original evaporated metal film in those regions where theintensity of incident light is greatest. As a consequence, there isdeveloped out of the original continuous evaporated metal film a visiblepositive image (i.e., one wherein the light-struck areas are light inhue compared to those areas which were not lightstruck).

The rate of metal removal is dependent upon the current density andduration. These values can vary within wide ranges. Generally, D.C.voltages less than 100 volts are used, and less than 50 volts ispreferred. When one considers that the path of current flow is not onlythrough the relatively low resistance path of the metal but also throughthe higher resistance of the binder wet by electrolyte the range ofuseful current densities from .1 to 5 milliamperes (or even higher),i.e., 1:50, is not surprismg. The range of high to low resistance pathsavailable before exposure is represented by optical transmissivity andis 40 to 70% or 1:1.75. The thickness of the zinc oxide layer is 0.1 to5 mils or 1 to 50 which represents the total length through which thecurrent must flow and whose resistivity is proportional to thethickness. The thickness of the metal film to be removed also varies.

Cobalt, titanium and vanadium metal films give the best quality imagesand are preferred. Titanium images are of relatively low opticalcontrast, but highly stable against fingerprints and high --humidity.Vanadiumimages display high resolution, superior stability and excellentcontrast.

Good optical contrast is generally obtained with cadmium films, withsomewhat limited stability against fingerprints and high humidity.However, cadmium images can be protected by spraying with an acrylicresin and then display indefinite resistance to high humidity,fingerprints and the like. Occasionally, poor contrast is obtained withcadmium films, apparently owing to the fact that the light yellowcadmium oxide formed during the oxidation process is sometimes notcompletely removed from the metal surface. Other metals are also usefuland can provide films of various colors, depending on the nature of themetal.

The invention is further described by reference to the accompanyingdrawing. FIGURE 1 is a diagrammatical illustration of the copy sheet ofthe invention, with the top layerthereof cut back to show anintermediate layer. FIGURE 2 is a diagrammatic representation of thecopy sheet of the invention, with a latent image present therein. FIGURE3 is a diagrammatic representation of the copy sheet after development,with an image appearing thereon. In the drawing, the relative thicknessof the layers of the copy sheet has been greatly exaggerated, forclarity.

Referring to FIGURE 1, numeral designates an electrically conductivecarrier, having a photoconductive layer ll bonded thereto. 14 designatesan intermediate nucleating layer of metallic atoms. 16 represents avapordeposited light-transmissive metallic layer.

In FIGURE 2, the multi-layer copy sheet construction of the invention,having electrically conducting base layer 10, photoconductive layer 12,intermediate nucleating layer 14 and light-transmissive metallic layer16, has been exposed to illumination over a portion of its surface, inthe form of an image as shown, using actinic radiation sufiicient tocause at least 10-fold increase in the conductivity of thephotoconductor in the area illuminated, the latent image being invisibleto the eye and existing as an area of increased conductivity in thephotoconductive layer. In this instance, the illumination was made usinga transparent mask in which the image of the numeral 3 was opaque, andthe remainder transparent, the result being that in the area designated18, conductivity of the photoconductive sheet remains the same as thatof the;

dark adapted layer, while the portion designated 19 has increasedelectrical conductivity.

FIGURE 3 shows the copy sheet of FIGURE 2 after development byelectrolysis, in which the image on photoconductive layer 12 consists ofthat part of light-transmissive layer 16 which has not been removed byelectrolytic oxidation of the metal, leaving a contrasting image 20corresponding to the latent image 18. In this instance, electrolyticoxidation was carried out using an electrolyte which effectively removedall of the metallic layer 16, but which converted the nucleating layer14 to an insoluble oxide of the metal employed for nucleation, whichremains upon the surface of the photoconductor 12 as a layer 22 which issubstantially invisible to the eye.

The invention is further illustrated by reference to the followingspecific examples, in which all parts are by weight unless otherwisespecified.

Example I A light sensitive copy sheet is prepared as follows: Asuspension of about 46 parts by weight of French Process Zinc Oxidemicrocrystals in a solution of 27 parts of toluene, 40 parts of acetone,and 11 parts of Pliolite S-7 (a resinous copolymer of 30 weight percentof butadiene and 70 weight percent styrene) serving as a binder, themixture having been ground in a ball mill until smooth, is wet-coated ona sheet of 1 mil. gauge aluminum foil which is backed with paper. Afterdrying, the firmly bonded smooth white coating of zinc oxide is found tobe about 0.8 mil. in thickness. The dried photoconductive layer of zincoxide is placed in a chamber of a vacuum coating apparatus, maintainingthe chamber under pressure of about 10 torr, and cleaned by glowdischarge in argon. The sheet is then prenucleated by vapor depositingan invisible coating of nichrome on the surface by electrically heatinga tungsten filament over which is suspended a U-shaped piece of nichromewire until the nichrome is at a temperature at which it vaporizessufficient nichrome being used to deposit 10 -10 atoms/ sq. cm. or about0.01 mg. for every sq. cm. of surface to be covered. Then, utilizing atest glass coated at the same time, and a photocell which reads theoptical transmission of the test glass with respect to a beam of lightwhich is passed through the glass and into the photocell, the surface ofthe photoconductive layer which was prenucleated with nichrome is coatedwith vanadium by evaporating vanadium (also conveniently from a vanadiumwire hung over a tungsten heater) until a layer of about 50 percentoptical transmissivity has been deposited. Surfaces thus prepared appearto be a medium gray color by visual inspection.

The copy sheet thus prepared is used as follows: An image is formed uponthe dark-adapted (24 hours) copy sheet, using a photographic negativebearing indicia to be copied. The copy sheet with negative held incontact therewith is exposed for about 5 seconds to 700-foot candlesillumination. The exposed sheet is immediately developed using anelectrolyte containing enough oxalic acid or potassium oxalate inaqueous solution to have sp. resistance=300 ohm-cm. The electrolyte isheld in a sponge which is made the cathode; the aluminum base layer ofthe photoconductive copy sheet is made the anode. A 60-volt directcurrent potential is applied between the metallic conductive base layerof the sheet and the sponge containing the electrolytic solution.

Development is carried out for 1 second at an averagecurrent densitysuflicient that approximately 60 millicoulombs of electrical charge persquare inch are used, A positive image, which can be viewed byreflection, is produced upon the surface of the copy sheet correspondingto the indicia projected thereon.

When development is carried out for 2 and 3 seconds, respectively,approximately 90 to millicoulombs are used. The images are of somewhatgreater contrast; however, oxidation carried out beyond the point atwhich all of the metal has been removed or oxidized in the illuminatedareas is obviously without additional effect.

The following table shows the results obtained when various metals areused as light-transmissive coatings on a zinc oxide photoconductivesheet. In each case, a transparent negative is employed, through whichthe sheet is exposed to light of the intensity and for the duration setforth. The electrolyte which is employed is dissolved in water to give asolution which at 25 C. has specific resistance of about 300-ohmcentimeters. The metalcoated photoconductive copy sheets aredark-adapted for periods ranging from 24 to 48 hours prior to exposure.The time of development of these exposed copy sheets ranges from onesecond to several minutes, depending upon the voltage and amperageemployed. Generally speaking, about 50-60 millicoulombs per squarecentimeter give good contrast. It is found that the use of highervoltage at shorter development times promotes improved contrast. In eachcase, a positive picture is obtained from the negative transparency.

While a sponge containing the aqueous electrolyte is conveniently madethe cathode, the exposed copy sheets can also be developed by immersingthem in an electrolyte solution contained in a shallow pan, with astainless steel cathode. The aluminum layer of the copy sheet is madethe anode. In the latter case, development can be observed as itproceeds, if desired.

TABLE I Opt. Development Metal 1 Trans, Exposure, Time, Remarks onVisual Observation percent Sec/F6 Sec.

Volt Milliarnps Electrolyte 30 20/150 Good contrast, gray tones. 50 /150Low contrast. 28 /150 Good contrast, dark gray tones. 30 15/150 Goodcontrast. 30 20/150 Good contrast, dark gray. 55 30/150 Fair contrast,light gray tones. 54 30/150 Do. 50 5/700 1 Excellent contrast. 48 3/7001 D 50 120/1 Low contrast. 50 120/1 180 D0.

1 A light transmissive coating of the metal named was vapor-coated onthe surface of a zinc oxide-pliolite photoconductive layer supported onan aluminum foil sheet backed with paper.

2 Exposed and developed using a commercially available microfilmreaderprinter; 8% inch wide sheet used; 11 inches passed under a spongewet with the electrolyte in 4.5 seconds for development.

3 Wiped by hand with a sponge wet with electrolyte and connected to abattery; metal backing of the copy What is claimed is:

1. A process for making a reproduction which comprises exposing to alight image a copy sheet comprising a substrate consisting of anelectrically conductive base sheet, an intermediate photoconductivelayer and a surface layer overlaying said photoconductive layerconsisting essentially of a light-transmissive metallic film havingabout 40 to 70 percent optical transmissivity, to create a latent imagein the photoconductive layer of the said copy sheet, wetting the surfaceof the copy sheet with an aqueous electrolyte solution, and creating adirect current electrical potential between the electrolyte as thecathode and the conductive base layer of the copy sheet as the anode toconvert the said light-transmissive metallic layer electrolytically to avisible record of the said light image.

2. A method according to claim 1, wherein the lighttransmissive metalfilm has about 50 percent optical transmission.

3. A copy sheet suitable for recording light images byelectrophotographic techniques, comprising an electrically conductivesubstrate, a layer of photoconductive material bonded to said substrate,comprising a particulate photoconductor and an insulating resinousbinder, and a lighttransmissive metal layer having about 40 to 70percent optical transmissivity adhered to the surface of saidphotoconductive layer.

4. A copy sheet according to claim 3, wherein the saidlight-transmissive metal is vanadium.

5. A copy sheet suitable for recording light images byelectrophotographic techniques, comprising an electrically conductivesubstrate, an intermediate photoconductive layer comprising aparticulate photoconductor and an insulating resinous binder bonded tosaid conductive sub strate, a nucleating layer consisting of about 10 to10 metal atoms per square centimeter distributed uniformly andadherently over the surface of said photoconductor, and alight-transmissive metal layer having about 40 to percent opticaltransmissivity distributed uniformly over and upon said nucleating metaland tightly adherent to said photoconductive layer.

6. A copy sheet according to cliam 5, wherein the nucleating metal atomsare nickel, and the light-transmissive film is vanadium.

References Cited by the Examiner UNITED STATES PATENTS 2,912,592 11/1959Mayer 96-1 X 3,010,883 11/1961 Johnson et al. 204-18 3,085,051 4/1963Hamm et al. 961 X 3,127,331 3/1964 Neher 96-1 X 3,127,333 3/1964 Bonrud96-1 X 3,199,086 8/1965 Kallman et al. 96-1 NORMAN G. TORCHIN, PrimaryExaminer. D. PRICE, R. MARTIN, Assistant Examiners.

1. A PROCESS FOR MAKING A REPRODUCTION WHICH COMPRISES EXPOSING TO ALIGHT IMAGE OF A COPY SHEET COMPRISING A SUBSTRATE CONSISTING OF ANELECTRICALLY CONDUCTIVE BASE SHEET, AN INTERMEDIATE PHOTOCONDUCTIVELAYER AND A SURFACE LAYER OVERLAYING SAID PHOTOCONDUCTIVE LAYERCONSISTING ESSENTIALLY OF A LIGHT-TRANSMISSIVE METALLIC FILM HAVINGABOUT 40 TO 70 PERCENT OPTICAL TRANSMISSIVITY, TO CREATE A LATENT IMAGEIN THE PHOTOCONDUCTIVE LAYER OF THE SAID COPY SHEET, WETTING THE SURFACEOF THE COPY SHEET WITH AN AQUEOUS ELECTROLYTE SOLUTION, AND CREATING ADIRECT CURRENT ELECTRICAL POTENTIAL BETWEEN THE ELECTROLYTE AS THECATHODE AND THE CONDUCTIVE BASE LAYER OF THE COPY SHEET